find source of input value modification on page ready in javascript

TL;DR is there's no easy way.

So working backwards from the code that was actually changing, I don't think there is a way with browser's debugging tools to put a breakpoint here. However with an educated guess about the dom node's property that was getting changed, you could (assuming you run soon enough in the page) set a breakpoint that would help you out.

You found that the code in question was $('#parent-div-id input').val('').

Now .val() in jQuery just finds the actual node element and sets node.value = "blah"; The important distinction here is that it was changing a "property" on the node, and not an "attribute". You'll notice that all the DOMAttribute etc breakpoints talk about node attributes. If you take a node and edit the html's attribute, by say doing $('#parent-div-id input').attr('value', 'blah'), the breakpoint would get hit. The list of properties on a node is much larger than the list of HTML attributes, and the debugging tools only put breakpoints on HTML attribute modifications.

However in this case, you know the element in question (the input element), and the property that was getting messed with (the value property), so you could have done what was suggested in this answer, and redefined the property.

Example code here:

var node = $('#parent-div-id input')[0];
Object.defineProperty(node, 'value', {
    get: function () {
        return node._value;
    },

    set: function (value) {
        debugger; // sets breakpoint
        node._value = value;
    }
});

All you'd need to do is get that code to run early enough in the page. To do that I'd probably set a breakpoint on the first line of the first javascript file loaded in the network tab.

Debug after insert this code on top line of your page.

setInterval(function() {
  var value = $("#report_sub_district_id").val();
  if(value !== 5123) debugger;
},10);

and, Maybe any Chrome extension program might cause the problem. You should disable your all Chrome extensions.